A customer, also called client, buyer, or purchaser, is usually used to refer to a current or potential buyer or user of the products of an individual or organization, called the supplier, seller, or vendor. This is typically through purchasing or renting goods or services. However, in certain contexts, the term customer also includes by extension any entity that uses or experiences the services of another. A customer may also be a viewer of the product or service that is being sold despite deciding not to buy them.

The word derives from "custom," meaning "habit"; a customer was someone who frequented a particular shop, who made it a habit to purchase goods of the sort the shop sold there rather than elsewhere, and with whom the shopkeeper had to maintain a relationship to keep his or her "custom," meaning expected purchases in the future.

The slogans "the customer is king" or "the customer is god" or "the customer is always right" indicate the importance of customers to businesses - although the last expression is sometimes used ironically.

However, "customer" also has a more generalised meaning as in customer service and a less commercialised meaning in not-for-profit areas. To avoid unwanted implications in some areas such as government services, community services, and education, the term "customer" is sometimes substituted by words such as "constituent" or "stakeholder". This is done to address concerns that the word "customer" implies a narrowly commercial relationship involving the purchase of products and services. However, some managers in this environment, in which the emphasis is on being helpful to the people one is dealing with rather than on commercial sales, comfortably use the word "customer" to both internal and external customers.